HELP! confusing background noise

I have put an MP3 player in my car and love it. I have a noise problem though. I have tried some filters and things and am about of ideas to get rid of it. I use a 140 Watt inverter with a computer and a normal power supply. I was hoping you might have an idea. It all sounds great when I run it off an extension cord from my house or when I run it off the inverter and use headphones from the computer. The weird thing is that when I do not plug the signal(sound) cable into my deck, the background noise is still there coming out of my car stereo. The rest of my car stereo has no problems ever though. It enclude a few amps if that makes and difference. Then I turn off the computer and leave the inverter on and the noise goes away. Some of these results seem to contradict each other. Any help would be appreciated. Thanks.